src/output-{proto,impl}.lisp: `sequencer''s :constraints initarg takes names.

src/output-{proto,impl}.lisp: `sequencer''s :constraints initarg takes names.

The `constraints' slot of a `sequencer' object maintains a list of
constraints, each of which is a list of `sequencer-item' objects, and
this list is ordered with the most-recently added constraint first.

Previously the :constraints initarg just set this list directly, which
isn't really very satisfactory.  Instead, handle the initarg specially,
reversing it, and converting item names into the actual items, interning
them properly.

This isn't completely right, because there's no way at this time to
attach handler functions to the implicitly created items, but it's way
better than nothing.